1. Field of the Invention
It is desirable to coat various substrates, but more particularly metal substrates, such as, for example, carbon steel, silicon steel and aluminum with a coating which will exhibit the following desirable properties:
A. Adhesion sufficient to withstand slitting, stamping, and rubbing. PA1 B. Good dielectric properties. PA1 C. Hardness which is acceptable but easy on the life of cutting dies. PA1 D. Temperature resistance to 1600.degree. F. or higher when the atmosphere is mildly oxidizing or reducing with the result that there is minimal deterioration of dielectric properties. PA1 E. Atmospheric corrosion resistance sufficient to prevent steel from rusting on prolonged exposure to plant atmosphere. PA1 F. Smooth coating texture to allow steel articles to slide over one another without dislodging the coating. PA1 a. Adhesion sufficient to withstand slitting, stamping, and rubbing. PA1 b. Good dielectric properties. PA1 c. Hardness which is acceptable but easy on the life of cutting dies. PA1 d. Temperature resistance to 1600.degree. F. or higher when the atmosphere is mildly oxidizing or reducing with the result that there is minimal deterioration of dielectric properties. PA1 e. Atmospheric corrosion resistance sufficient to prevent steel from rusting on prolonged exposure to plant atmosphere. PA1 f. Smooth coating texture to allow steel articles to slide over one another without dislodging the coating. PA1 when (C) is zero, then (D) is at least 1% and (F) is at least 0.5%; PA1 when (D) is zero, then (C) is at least 1% and (F) is at least 0.5%; PA1 when both (C) and (D) are each zero, then (E) is at least about 0.9%; PA1 when (C) and (E) are each zero, then (D) is at least 1% and (F) is at least 0.5%; PA1 when (D) and (E) are each zero, then (C) is at least 1% and (F) is at least 0.5%; PA1 when (C) is zero, then (D) is greater than zero but less than 1% when (E) is at least 0.9%; PA1 when (D) is zero, then (C) is greater than zero but less than 1% when (E) is at least 0.9%; PA1 the concentration of phosphoric acid is that stoichiometric quantity which is required to react with either (C), (D) or (E), or with mixtures thereof to form silicic acid; and the ratio of M.sub.2 O to SiO.sub.2 is from about 1:1 to about 2:1. PA1 when (C) is zero, then (D) is at least 1% and (F) is at least 0.5%; PA1 when (D) is zero, then (C) is at least 1% and (F) is at least 0.5%; PA1 when both (C) and (D) are each zero, then (E) is at least about 0.9%; PA1 when (C) and (E) are each zero, then (D) is at least 1% and (F) is at least 0.5%; PA1 when (D) and (E) are each zero, then (C) is at least 1% and (F) is at least 0.5%; PA1 when (C) is zero, then (D) is greater than zero but less than 1% when (E) is at least 0.9%; PA1 when (D) is zero, then (C) is greater than zero but less than 1% when (E) is at least 0.9%; PA1 the concentration of acid is at least the stoichiometric quantity required to react with either (C), (D), or (E), or with mixtures thereof to form silicic acid; and the ratio of M.sub.2 O to SiO.sub.2 is from about 1:1 to about 2:1 in said alkali metal metasilicates and orthosilicates. PA1 1. Placing the article in a dip bath containing the hydrosol and maintaining a fixed hydrosol composition in the dip bath by continuously or intermittently replacing consumed hydrosol. The article may be cleaned prior to coating (if not already coated) by washing in dilute acid or alkali. PA1 2. Passing the article to be coated through the dip bath at a constant rate and providing PA1 proper drainage of wetted articles in order to avoid excess accumulations of liquor on the article surfaces. PA1 3. Passing the article through a ventillated dryer where it is exposed to a temperature of about 700.degree. F. for 30 seconds or the equivalent thereof.
It is also desirable that said coating have the capability of being deposited instantaneously and uniformly from an aqueous system upon contact of said aqueous system with a substrate.
2. Description of the Prior Art
Several patents have been issued which disclose the use of silica or silicates and phosphate coatings for steel. For example, U.S. Pat. No. 3,138,492 discloses adherent, electrically insulating coatings when applied to iron strips or sheets. The coatings are formed from aqueous slurries containing phosphoric acid and 2 to 30% colloidal SiO.sub.2.
Other patents which disclose the use of colloidal silica are U.S. Pat. No. 3,650,783 and Japanese Patent No. 7,406,742. In U.S. Pat. No. 3,650,783, the cation supplied is C.sub.r.sup.+.sup.3 which provides a composition quite different from the base coating of commercially available steel.
Japanese Pat. No. 7,406,742 discloses a phosphoric acid system to which an aluminum oxide solution and colloidal silica are added in combination.
U.S. Pats. Nos. 3,301,701 and 3,522,113 concern the direct addition of alkali metal silicates as coating compositions in an alkaline system.
Other patents which disclose the use of silicates which are not in a hydrosol form are German Pats. Nos. 1,264,924 and 2,347,728.
U.S. Pat. No. 3,214,302 discloses the application of an alkali metal silicate base coat on steel sheet followed by a top coat of phosphate or sexivalent chromium applied from an aqueous solution.
U.S.S.R. Pat. No. 157,587 discloses treatment of a steel surface with magnesium oxide followed by coating with an aqueous solution of phosphoric acid, or a phosphoric acid solution which may contain (NH.sub.4).sub.3 PO.sub.4, Mg(H.sub.2 PO.sub.4).sub.2 or Al(H.sub.2 PO.sub.4).sub.3.
None of the above prior art patents provides an acidic hydrosol which has the property of instantaneously depositing a substantially uniform coating upon a variety of substrates when placed in contact therewith and which coating exhibits the desirable properties aforementioned.
It is, therefore, an object of the present invention to provide an acidic aqueous system which is capable of depositing a substantially uniform coating instantaneously upon a substrate when placed in contact therewith, said substrate being one which can consume minor quantities of acid from said system.
It is another object of this invention that said coating be one which exhibits the following desirable properties.
It is a further object of this invention that the coating be of a relatively uniform weight per unit of substrate area and that said coating weight be independent of the time of contact between the aqueous system and the substrate.
It is another object of this invention that said coating be firmly bonded to the substrate and that when the coating is dried and subsequently heated it provides suitable dielectric properties.
It is another object of this invention that the coating not be reduced by the silicon present in silicon steels during stress relief annealing and that the coating not evaporate at the annealing temperatures conventionally employed in the art.
It is a still further object of this invention that said coatings can be stress relief annealed on soft magnetic silicon steels without the release of excessive quantities of water vapor which are deleterious to magnetic qualities.
It is yet another object of this invention that said coating can be applied successively to said substrates without the loss of the aforementioned desirable properties.
It is a further object of this invention that said coating act as a separator when coated steel articles are placed in laminated stacks and are stress relief annealed.